This invention relates generally to the manufacture of integrated circuit devices, and more specifically, to the deposition of epitaxial films by chemical vapor deposition.
In a chemical vapor deposition (CVD) system, source gases such as SiH4 and GeH4 are injected into a chamber and react on heated wafer surface, thereby depositing or growing a film on top of the wafer. When a single-crystal film is deposited or grown on a single-crystal substrate, the crystal growth is called epitaxy. For high quality epitaxial film growth, it is important to use a system with a high purity environment, i.e., with ultra-low background of impurities such as H2O, O2, and other oxygen-containing species. If the system used does not achieve this high purity condition, the film grown will have poor quality. Specifically, it has been found that epitaxial films grown in an environment containing the above-mentioned impurities often exhibit a rough surface or have a high density of pits.
CVD chambers often are not able to achieve this requirement of ultra low impurity level, either because of limitations of the system or contamination of the chamber or process gases. Bringing the system to ideal high-purity conditions typically requires changing the deposition tube, a thorough bake out, or purchase of a new system. All of these options are time consuming and costly.
Various approaches to prevent or remove contamination have been attempted. In one approach, a film or coating is applied to prevent particles or impurities from being sputtered off from a member in the reaction chamber. For example, U.S. Pat. No. 6,374,871 to Donohoe discloses a removable container which is inserted into a processing chamber and in which the work piece processing is carried out. The container includes one or more ports located in the side and connecting with ports of the processing chamber which provide gases or other materials used in processing. Donohoe teaches that the use of such container prevents particles releasing from the process chamber, and reduces the need to periodically clean processing chambers. However, such container does not address the problem of the molecular contamination such as H2O, O2 and other oxygen-containing species present in the chamber or introduced by source gases.
In another example, U.S. Pat. Nos. 6,120,660 and 6,217,724 to Chu et al disclose the use of a silicon coating on the wafer susceptor or as a liner on the chamber walls in a plasma immersion ion implantation treatment system, and U.S. Pat. No. 5,134,301 to Kamata et al. disclose a silicon coating on ion implanting apparatus such as an electrode. Such coatings protect the wafer from impurities dislodged from the susceptor, chamber walls or electrode during processing. However, these coatings do not address the problem of the molecular contamination such as H2O, O2 and other oxygen-containing species present in the chamber or introduced by source gases.
Another approach involves the use of a coating of polycrystalline silicon, or polysilicon, on the wafer itself. Such polysilicon coatings act as gettering members to remove contaminants after they are already present in the substrate. For example, U.S. Pat. No. 5,374,842 to Kusakabe discloses the use of a polysilicon coating at the backside of the wafer to getter metals entering the wafer front surface during processing, thereby removing metals from the active device area so that device performance will not be impacted. In another example,.U.S. Pat. No. 4,053,335 to Hu discloses a backside layer of polysilicon for gettering of unwanted impurities from the integrated circuits. However, such polysilicon gettering of contaminants after they are already present in the substrate does not solve the problem of contaminants in the processing chamber adversely affecting the quality of epitaxial film growth, especially in a CVD system.
Thus, there remains a need in the art for a method of achieving a high-purity environment and preventing contaminants in the CVD processing chamber from reaching the wafer surface during epitaxial silicon growth.
It is therefore an object of this invention to provide an improved method for forming an epitaxial layer on a monocrystalline substrate. In one aspect of the invention, a method is disclosed for forming an epitaxial layer on a front side of each of a plurality of substrates, the front side being formed of a monocrystalline material. The method comprises the steps of: providing a plurality of gettering wafers formed of a gettering material; arranging the plurality of substrates and plurality of gettering wafers in a chemical vapor deposition system, such that the front side of each substrate is facing one of the plurality of gettering wafers; and forming the epitaxial layer by chemical vapor deposition. Impurities present in the chemical vapor deposition system during formation of the epitaxial layer are gettered by the gettering wafers.
In another aspect of the invention, a method is disclosed for forming an epitaxial layer on a front side of each of a plurality of substrates, the front side being formed of a monocrystalline material and a back side of each of the plurality of substrates comprising a layer of a gettering material. The method comprises the steps of: arranging the plurality of substrates in a chemical vapor deposition system, such that the front side of each substrate is facing the back side of another of the substrates; and forming the epitaxial layer by chemical vapor deposition. Again, impurities present in the chemical vapor deposition system during epitaxial formation are gettered by the layer of gettering material.
In yet another aspect of the invention, a method is disclosed for forming an epitaxial layer on a front side of a substrate, the front side being formed of a monocrystalline material. The method comprises the steps of: depositing a layer of a gettering material on an interior surface of a chemical vapor deposition system; and forming the epitaxial layer by chemical vapor deposition. Again, impurities present in the chemical vapor deposition system during epitaxial formation are gettered by the layer of gettering material.